Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration. Measurements made in a borehole (i.e., down hole measurements) are typically performed to attain this understanding, to identify the composition and distribution of material that surrounds the measurement device down hole. To obtain such measurements, logging tools of the acoustic type are often used to provide information that is directly related to geo-mechanical properties.
Traditional acoustic tools utilize transmitters to create pressure waves inside the borehole fluid, which in turn create several types of waveguide modes in the borehole. Corresponding modes of propagation occur in the formation surrounding the borehole, and each of these can be used to provide information about formation properties. Thus, once data associated with the various modes is acquired, it can be processed to determine formation properties, such as compression and shear wave velocity in the formation. For this reason, acoustic tools are an integral part of modern geophysical surveys, providing information on the mechanical properties of the medium by measuring acoustic modes of propagation.
Conventional acoustic tools rely on discrete, mechanically independent sets of receivers, where one electrical measurement is made per moving part (e.g., per receiving transducer). Since acoustic systems transform the signal from the spatial domain to the velocity domain, the spatial separation between individual sensors determines the maximum unambiguous velocity that can be measured, as dictated by the Nyquist-Shannon sampling theorem.
In these conventional tools, at least two difficulties arise. First, velocity measurement becomes non-unique, due to aliases observed in both time and frequency analysis. Second, signals arriving at different sensors are related to each other in a non-trivial way. Relatively complicated analysis methods, such as semblance processing, are used in an attempt to deconstruct relationships between them.
Although these difficulties can be reduced somewhat by using more sensors, and spacing them more closely together, this approach may be difficult to realize due to mechanical constraints, and increased overall cost.